The Science Behind… experimenting with sourdough starters

A few days ago, I found myself without all-purpose flour or bread flour (I blame Preston Yancey’s white bread – it requires 2.5 lbs of flour each time I make it!). That’s not too big of a deal for my bread, since I can always switch to making a 100% whole wheat.

But this time? I had a sourdough starter to feed!

When I feed my starter, I tend to discard nearly all of it, and only feed it enough for the next batch of dough I need it for. That means, for my 1-2-3 bread, I need 120g of starter. So I’ll add 75g of flour and 60g of water and that’s it.

I added the rye flour and hoped for the best. After letting it sit overnight, the starter seemed active, I added it to my dough, and the bread came out perfectly, with a hint of rye flavor.

It got me thinking – is there a difference between a starter made with white, whole wheat or rye flour? Can I feed the starter with any type of flour I want, and are the flours interchangeable?

I decided to find out. First though, a little refresher on what starter is and what it does.

What we call sourdough starter is a combination of wild yeasts (Saccharomyces exiguus, as opposed to saccharomyces cerevisiae found in instant yeast or active dry yeast) and the bacteria lactobacillus acidophilus. The yeasts and lactobacillus culture live in a symbiotic relationship where the yeasts consume sugars that are broken down by enzymes in the flour, and the lactobacillus regulate the speed at which the enzymes break down the sugars so the yeast always has something to eat and doesn’t release carbon dioxide too quickly, before the gluten has time to form a strong net to trap the gases.

So, what happens if the enzymes break up all the starches into sugars? If all the starches turn into sugars, the yeast organisms feed off the sugars in flour and reproduce quickly. If they grow too numerous, they can exhaust the amount of sugar in the flour and will look for other sources of food – namely, gluten. This is particularly bad because they don’t convert back once sugars are reintroduced. Which means, if you add a transformed starter to a regular dough (one that now chows down exclusively on gluten), the dough will start forming gluten beautifully, and look great, until the yeast eat up all the gluten, at which point the dough becomes a runny mess and you can’t do anything with it except throw it out.

Make sense? We want to make sure the enzymes are slowed down before this happens. Enter lactobacillus acidophilus.

lactobacillus is also present in yogurt

The lactobacillus isn’t that important in white or whole wheat breads, because wheat flours don’t contain a whole ton of amylase enzymes (the ones that break down the starch into sugars).

Rye flour, on the other hand, has lots of amylase. Lots of smarter bakers than me agree that rye breads turn out better when they are made with sourdough starter because it regulates the amylase enzymes and prevents it from turning all the starches into sugar which results in rubbery, mushy bread.

So, is good rye bread only possible with rye starter?

Nope. I’ve had good luck with sourdough starters made from all-purpose flour, whole wheat flour, and rye flour. The only difference it makes is if you want a 100% rye bread.

What I’ve found from my own experience and corroborated by strangers on the internet (here, here and here) is that the flour you use to feed your starter has very little impact on its overall yeast composition and starch to sugar enzyme action.

Where it does make a difference is in the taste. If it makes up a substantial portion of your finished dough, (by that I mean more than 20% by weight) it can affect the taste of your dough, but only in a good way. Breads taste better when they are a mix of different flours – it adds a little more complexity.

So feel free to experiment with your sourdough starter when you feed it. It’s super easy to do. After I’ve used up most of my starter, I’ll just feed it again with a different flour, and it comes back to life just as vigorously as before.

That’s the science behind… using different flours in your sourdough starter. If there’s something you didn’t find clear, please let me know in the comments!